As white LEDs with higher luminance have recently been developed, LEDs are increasingly used not only for display, but also for lighting.
A typical LED chip has the following construction. A multilayer epitaxial structure including a light emitting layer is formed on a single-crystal substrate such as a sapphire substrate. An anode electrode and a cathode electrode are provided on a surface of the multilayer epitaxial structure which faces away from the sapphire substrate. Such an LED chip can not be used by itself, and therefore mounted on a printed wiring board or the like.
When used for display, this LED chip is generally mounted in such a manner that the multilayer epitaxial structure is faced up (that is to say, the sapphire substrate is faced down). This mounting method is hereinafter referred to as epitaxial-side-up bonding or up-bonding. Power is supplied to the LED chip through metal wires connected to the anode and cathode electrodes, so that the LED chip emits light.
However, the up-bonding mounting method poses a problem related to heat when this LED chip is used for lighting. To use the LED chip for lighting, considerably larger amounts of currents need to be applied to obtain a significantly larger luminous flux, when compared with a case of using the LED chip for display. Therefore, unless heat dissipation is sufficiently performed, a luminous efficiency is worsened and a lifetime is shortened.
To solve this problem, when used for lighting, the LED chip may be flip-chip mounted to assure sufficient heat dissipation. To be specific, the LED chip is mounted on a highly heat-dissipative substrate having a bonding pad, by means of a metal bump, for example, a gold (Au) bump, in such a manner that the multilayer epitaxial structure is faced down (that is to say, the sapphire substrate is faced up). In this way, the light emitting layer that mainly generates heat is positioned closer to the highly heat-dissipative substrate. Furthermore, the flip-chip mounting method enables the heat to be conducted to the highly heat-dissipative substrate through the metal bump. As a consequence, better heat dissipation is achieved, so that the LED chip is prevented from being heated excessively.
In this case, however, light produced by the light emitting layer is emitted outside through the sapphire substrate. Therefore, a light extraction efficiency is lowered, when compared with a case where the LED chip is up-bonded and light produced by the light emitting layer is therefore emitted outside without going through the sapphire substrate. This problem may be solved by removing the sapphire substrate from the LED chip that is to be flip-chip mounted. This solution, however, makes it difficult to handle the LED chip, since the multilayer epitaxial structure has a thickness of only approximately 5 μm.
“D. Morita, et al, Jpn. J. Appl. Phys. Vol 41 (2002) pp. L1434-L1436” discloses an LED chip that improves heat dissipation without lowering a light extraction efficiency. According to this technique, a cathode electrode is formed on part of an upper surface of a multilayer epitaxial structure, and an anode electrode is formed on the entire lower surface of the multilayer epitaxial structure. Here, a metal layer is formed so as to be in contact with the anode electrode. In other words, the metal layer is provided as a replacement for a sapphire substrate which has been removed (lift off), for reinforcement. Hereinafter, this LED chip is referred to as a lift-off LED chip.
The lift-off LED chip is mounted on a mounting substrate in such a manner that the metal layer is faced down so as to be connected to a bonding pad provided on the mounting substrate by using a solder or a silver paste. Power is supplied to the lift-off LED chip through a metal wire connected to the cathode electrode and the bonding pad, to cause the lift-off LED chip to emit light. The lift-off LED chip does not have a sapphire substrate on or above the multilayer epitaxial structure, more specifically, above a light extraction surface of the multilayer epitaxial structure. This improves a light extraction efficiency. Furthermore, the multilayer epitaxial structure is mounted on the substrate with the anode electrode and the metal layer therebetween. This achieves better heat dissipation.
According to the lift-off LED chip, however, shadow of part of the metal wire which is positioned above the light extraction surface appears at a light-irradiated surface when light is emitted. This causes a serious problem when using the lift-off LED chip for lighting. It should be noted that the up-bonding mounting of the typical LED chip has the same problem.
In light of this problem, an object of the present invention is to provide a semiconductor light emitting device that can (i) achieve high heat dissipation without lowering a light extraction efficiency and (ii) emit light that is not likely to produce shadow of a metal wire on a light-irradiated surface. The object also includes provision of a manufacturing method of the semiconductor light emitting device, and a lighting module and a lighting apparatus including the semiconductor light emitting device.